The present description relates to an internal combustion engine, more specifically to an internal combustion engine having a variable valve lift mechanism.
A continuously variable valve lift (CVVL) system is known and presented, for example, in U.S. Patent Application Publication US2005/0139183A1. The CVVL system has a drive shaft, which is equivalent to a conventional camshaft driven by an engine crankshaft, a cam which oscillates and pushes the intake valve against a return spring to open it, and a link mechanism which converts the rotational movement of the drive shaft to the oscillating movement of the cam. The system also has an electric motor which actuates a control shaft. The conversion between the rotational and oscillating movements varies in dependence on an angular position of the control shaft. Therefore, by rotating the control shaft with the electric motor a valve lift characteristic may be controlled as desired. Generally, in a CVVL system, as a valve lift becomes greater, a valve opening timing advances and a valve closing timing retards.
When an internal combustion engine starts its operation in an engine warmed-up condition, temperatures inside the engine combustion chambers or cylinders increase. During engine cranking, a piston in the cylinder moves relatively slowly and air is inducted and stays longer in the cylinder. Therefore, the air is heated further over the course of the intake and compression strokes of a cylinder cycle. If a greater amount of air is charged into the cylinder, particularly, when a geometric compression/expansion ratio is greater for a greater efficiency of the engine, the air is compressed during the compression stroke and the cylinder air temperature may become too high to combust the air and fuel in an intended manner. An unintended manner of combustion may include, for example, pre-ignition and knocking. These may lead to quality or reliability issues of the engine.
In the '183 publication, at the beginning of an engine cranking, the intake valve lift is minimum and the intake valve closes before an intake bottom dead center of a cylinder cycle. After some period has passed while cranking the engine, the intake valve lift is increased and the intake valve closes around the intake bottom dead center. Since the intake valve closes substantially before the intake bottom dead center at the beginning of the engine cranking, the cylinder air charge at the intake or compression bottom dead center is reduced and the cylinder air temperature during the compression stroke may decrease to some extent. However, there may still be a need to decrease the cylinder air temperature at the beginning of engine cranking, for example, in a case where an engine has a greater compression ratio and it is in a warmed-up condition.